As one of the methods for controlling internal combustion engines, there is known the method which determines a manipulated variable of each actuator with torque and an air-fuel ratio used as controlled variables. For example, Japanese Patent Laid-Open No. 2010-7489 discloses the method which determines required torque and a required air-fuel ratio for an internal combustion engine, and determines the respective manipulated variables of a throttle, an ignition device, and a fuel injection device in order to realize the requirements. In regard with the throttle, a throttle opening degree which is a manipulated variable of the throttle is determined in accordance with the target air amount for realizing the required torque. For example, by using a reverse model of an air model, the throttle opening degree required for realizing the target air amount can be obtained by calculation.
Incidentally, an air-fuel ratio is also closely related to the torque generated by an internal combustion engine in addition to the air amount which is taken into cylinders. In the case of the same air amount, if the air-fuel ratio of the air-fuel mixture provided for combustion is leaner than stoichiometry, the torque is decreased, and if the air-fuel ratio of the air-fuel mixture is rich, the torque is increased. Therefore, in the process of converting required torque into a target air amount, the air-fuel ratio of the air-fuel mixture in the cylinder, that is, the required air-fuel ratio is desirably referred to. By setting the target air amount in accordance with the air-fuel ratio requirement, realization precision of the required torque can be enhanced.
However, a required air-fuel ratio is not always constant, and is sometimes positively changed from the viewpoint of the exhaust gas performance. For example, at the time of recovery from fuel cut, in order to restore the NOx reducing ability of a catalyst quickly, the required air-fuel ratio is made far richer than stoichiometry for a predetermined time period. Further, in order to enhance purification performance of the catalyst, the required air-fuel ratio is periodically changed with stoichiometry as the center, and the required air-fuel ratio is also changed by air-fuel ratio feedback control. In these cases, the target air amount also changes in accordance with a change in the required air-fuel ratio, and the throttle opening degree is controlled in accordance with it. The movement of the throttle at this time is such a movement as to cancel out the torque variation accompanying the change in the air-fuel ratio by an increase or a decrease of the air amount. That is to say, when the air-fuel ratio is changed to the rich side, the throttle is moved to the closing side so as to cancel out the increase in torque due to the change by a decrease in the air amount. In contrast with this, when the air-fuel ratio is changed to the lean side, the throttle is moved to the opening side so as to cancel out the decrease in torque due to the change by an increase in the air amount.
However, there is a delay in the response of the air amount to the movement of the throttle, and the actual air amount changes later with respect to the change in the target air amount. Therefore, when the required air-fuel ratio is suddenly changed, the change of the air amount does not catch up with the change in the required air-fuel ratio. As a result, problems as follows occur.
FIG. 7 is a diagram showing in a chart, a change with time of each of torque, a rotational speed, an air-fuel ratio, a fuel injection amount, a cylinder intake air amount, and a throttle opening degree when a required air-fuel ratio is suddenly changed. In the chart of each stage, the dotted line shows the change with time of the required value or the target value of each item, and the solid line shows the actual behavior of each item. As shown in the drawing, when the required air-fuel ratio is suddenly changed to the lean side stepwise, the target air amount is suddenly increased stepwise correspondingly. However, the throttle opening degree cannot be increased stepwise, there is a delay in the response of the air amount with respect to the movement of the throttle, and therefore, the actual air amount is increased later than the target air amount.
The fuel injection amount is determined by the actual air amount and the required air-fuel ratio, and therefore, the fuel injection amount is temporarily decreased significantly due to the delay of increase of the air amount. As a result, the torque generated by the internal combustion engine is temporarily reduced significantly to be smaller than the required torque, and the engine speed is also temporarily reduced significantly. With this, a variation also occurs to the actual air-fuel ratio. According to the art described in Japanese Patent Laid-Open No. 2010-7489, when a deviation can occur between the actual torque and the required torque, the ignition timing is regulated to compensate for the deviation. However, when the ignition timing is set at the optimal ignition timing, it is difficult to increase the torque more than the torque at the optimal ignition timing, though the torque can be reduced by retardation of the ignition timing. Therefore, when the required air-fuel ratio is suddenly changed to the lean side, temporary reduction in the torque and the rotational speed as shown in FIG. 7 occurs.
More specifically, in the aforementioned conventional control method, not only the driving performance is likely to be impaired by variations in the torque and the rotational speed, but also reduction in emission performance is likely to be caused by the unintended variation in the air-fuel ratio as a result.